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CARBON DIOXIDE (CO2) ENRICHMENT – GASEOUS GOLD

 

Big Yields do come in Bottles!

CO2-gear

 

Excerpt from Integral Hydroponics Evolution by G.Low (Coming soon!)

 

To repeat some material, biomass is the amount of organic matter, such as plant tissue, found at a particular time and place. The rate of accumulation of biomass is termed productivity. Primary production is the rate at which plants produce new organic matter through photosynthesis. In order for photosynthesis to occur the plant needs light energy, chlorophyll, CO2, water and nutrients.

 

CO2 plays the most important role in biomass production because more than 90% of dry matter of living plants is derived from photosynthetic CO2 assimilation.[1] Plants use the carbon from CO2 and convert it into carbon compounds such as glucose, carbohydrates, lignin, and cellulose which is what becomes the biomass of the plant. This is why by increasing the levels of CO2 in the grow room we are able to increase the rate of photosynthesis, which increases the rate of biomass production. In simple terms more CO2 means more available carbon for the plant to use in biomass production.

 

Atmospheric CO2 levels tend to vary between approximately 350 – 500 ppm. These levels differ because of locality and contributing environmental factors. By increasing the naturally occurring atmospheric levels of CO2 in the grow room it is possible to increase the rate of photosynthesis through the so called CO2 “fertilizer” effect. This means that cell division occurs more rapidly which means plant growth/biomass production occurs more rapidly. Elevating CO2 levels in the grow room can increase yields by approximately 20-30% and reduce the growing time by 10 – 30%. This makes CO2 the magic bullet (‘gaseous gold’) that all growers seek.

 

It is interesting to note that the earliest land plants were established in Mid-Palaeozoic era at high CO2 (1500–3000 ppm) before the beginning of a period of low CO2 (<1000 ppm).[2] Therefore, many plants evolved in conditions of higher CO2 than are currently seen as atmospheric CO2 norms today (350-500ppm). As such, one way of looking at things is that CO2 enrichment isn’t so much a case of additional CO2 somehow supercharging plants but more a case of plants performing better under the conditions in which they evolved. Thus, one argument that could be put forward is that the true genetic potential of many C3 plants can only be realized in CO2 enriched environments (i.e. in environments similar to that in which they evolved).

 

Among other things, besides increased growth rates and yields, plants that are grown in CO2 enriched environments have larger root systems, thicker stems and larger leaves. This adds up to a stronger and sturdier plant.

 

Additionally, studies indicate that elevated CO2 levels increase trichome production, flavonoid and phenolic content in some species of plants.[3], [4] This means that elevated CO2 can potentially improve the resin/essential oil production, flavor and quality of some crops.

 

One other potential benefit to elevated atmospheric levels of CO2 for growers who use bennies for root disease prevention is that elevated CO2 levels result in more carbon input by the roots of the plant to the rhizosphere. This extra carbon input is believed to stimulate the growth of rhizosphere microorganisms.[5] Responses of soil-borne microflora to elevated atmospheric CO2 are different for various bacteria and fungi; however, some efficient rhizosphere colonizing beneficial bacteria (e.g. Pseudomonas) and fungi (e.g. Trichoderma spp.) are shown to benefit from elevated levels of atmospheric CO2. [6] In other words, like many C3 plants, some beneficial bacteria and fungi thrive under conditions of elevated CO2.

 

To give you some idea of how effective CO2 enrichment is, today 80-90% of commercial greenhouses in the Netherlands are CO2 enriched. Yield increases of 30-40% for some crops are not uncommon where CO2 levels are maintained at 3 times ambient levels (near 1000ppm).[7]

 

CO2, Photosynthesis and Photorespiration

 

To dumb things down somewhat, plants consume CO2 and release oxygen during the day as part of the process of photosynthesis (carbon dioxide + water → sugar + oxygen). At night they consume oxygen but don’t release oxygen. Instead they release CO2 as part of the process of respiration (sugar + oxygen → carbon dioxide + water). Therefore, plants require CO2 during the day for photosynthesis and oxygen at night for respiration.

 

During photosynthesis CO2 enters the plant through small openings in the leaves called stomata, is then captured or ‘fixed’ by photosynthetic enzymes and is then converted into carbohydrates. When atmospheric CO2 concentration goes up more CO2 will enter the leaves of plants (photosynthetic/growth rates will increase) because of the increased CO2 gradient between the leaf and the air.

 

Of even more importance is that an increase of CO2 concentration inhibits photorespiration which is the process where oxygen is absorbed and CO2 is released by the plant. At current atmospheric CO2 levels photorespiration can reduce the net carbon gain from photosynthesis by as much as 50%.[8]

 

The biochemistry of photosynthesis differs greatly amongst plant species; however, C3 plants (e.g. tomato. cucumber, pepper) particularly can benefit from increased atmospheric CO2 levels. See the following graph which shows the percent change of photosynthesis at 680ppm compared to 300-350ppm (atmospheric) CO2 for several C3 plant species.

 

CO2-photosynthesis-graph

 

Source: Agricultural Dimensions of Global Climate Change. Kaiser, H. M. Drennen, T. E. (Eds). St.Lucie Press: Florida, 1993

 

If CO2 is ‘Gaseous Gold’ Why Aren’t All Indoor Growers Using It?

 

Firstly, most top shelf (advanced/expert) growers with commercial outcomes in mind are using CO2 enrichment as part of their growing regime.

 

Secondly, many have tried and got it wrong for various reasons., e.g. they wanted to do it on the cheap and failed to invest in the right equipment, got bad advice from the outset, or attempted to run a CO2 enriched environment while their skill set was out of range to this level of grow room sophistication.

 

Other than this…

 

Creating a CO2 enriched environment can be an expensive and complex business. Obviously, if it were cheap and easy then everybody would be reaping the rewards of CO2 enrichment.

 

The Complexities of Creating a CO2 Enriched Environment

 

Let’s consider a few problems when it comes to running a CO2 enriched grow room.

 

Firstly, CO2 gas is expensive to purchase as compressed bottled gas. As such we don’t want to waste CO2 by venting large amounts of it outdoors. Therefore, in order to maintain CO2 at high levels in the grow environment for extended periods of time it is necessary to contain the CO2 enriched air within that environment. Because of this exhaust fans need to be turned off for extended periods of time or, not used at all. In addition, any possible leakage points need to be sealed to prevent CO2 gas passively leaking from the grow room through gaps in floors, under doors etc.

 

Herein lies part of the problem when dealing with the complexities of creating a CO2 enriched environment. For the CO2 to remain at optimum levels within the environment for extended periods of time airflow in and out of the environment must be significantly limited or, stopped completely. This means that the artificially enriched CO2 environment must be contained because any leakage and air extraction will result in rapidly decreasing levels of CO2.

 

However, let’s consider the average indoor grow room that isn’t CO2 enriched. Firstly, we have HID lighting that creates large amounts of heat. Secondly, we have exhaust fans that deal with this heat through extracting hot air out of the environment and replacing it with cooler outside air. Excessive heat reduces photosynthesis/growth and if the grow room environment becomes over-heated; not only can this greatly stress plants but kill them.

 

Additionally, air venting also deals with humidity. Large healthy growing plants release high levels of water vapour into the atmosphere through transpiration and this creates high humidity in the grow room. High humidity stops the plant from transpiring and photosynthesis slows or stops. High humidity also causes other problems such as creating an ideal environment for fungal pathogens (e.g. Botrytis) to thrive. Therefore, air ventilation (removing humid air from the grow room and drawing less humid air in from outdoors) is typically the means that growers employ to combat rising humidity in the grow room.

 

Given this, venting the grown room through inlet and outlet fans acts on two very important levels where environmental control is concerned; 1) venting removes heat emitted from lamps and cools the grow room and, 2) venting removes humid air from the grow room and replaces it with less humid air, drawn from outdoors.

 

However, where creating a CO2 enriched environment is concerned venting must go because venting will also remove the all-important CO2 from the grow room. Therefore, we need to find other ways of controlling heat and humidity in the grow room.

 

What this comes down to is employing equipment such as air cooled lights or tubes (to remove lamp heat prior to it entering the grow room environment), split system air conditioning (to cool the environment) and dehumidifying units (to remove moisture/water/humidity from the grow room air).

 

CO2 Grow Room Configuration/Design

 

There are two CO2 enriched grow room configurations that are commonly used by growers. One is a fully sealed environment where venting is never used. Typically, this grow room configuration is referred to as a ‘sealed’ or ‘enclosed’ CO2 grow room.   The other is a grow room that can be vented. In other words, the latter grow room is equipped with inlet and outlet fans which enables periodic venting. I refer to this configuration as an ‘intermittently night vented CO2 grow room’.

 

What I will do now is put up a single CO2 enriched grow room design that covers both of these approaches. See following image….

 

CO2 Growroom

 

CO2 is heavier than the other constituents found in air. You will note in the grow room configuration pictured that a CO2 delivery hose line is placed over the tops of the lamps, well above the plants. This ensures the gas is delivered above the plants whereupon it falls directly onto/over the plants. Other than this, you will see a makeshift CO2 redelivery system on the floor that ensures CO2 is circulated at ground level back up to/through the plants.

 

This redelivery system is simply a piece of ducting with a fan attached one end and the other end blocked to stop the exit of air. I then punch some holes in the top of ducting so the air/ CO2 is pushed upwards from the floor to the plants leaf undersides (directly to the stomata).

 

Additionally, there is an pedestal fan in the room to keep air moving around. Air movement ensures that the area directly around the plants doesn’t become CO2 depleted while also ensuring the CO2 sniffer/doser unit is monitoring levels of CO2 that are accurate to the room value as a whole. I.e. air movement ensures CO2 is evenly distributed throughout the plants and the room.

 

Additionally, the grow room configuration utilizes fan baffles to stop air/CO2 leakage when the fans are off. Therefore, this grow room design can be used for both a fully sealed/enclosed environment (where air extraction isn’t used at all) or an intermittently night vented environment where periodic air extraction can be used for reasons I’ll discuss in a moment. See following image of a fan baffle…

 

FanBaffle

 

Fan baffles open when the fans come on and close when the fans are turned off. This acts to seal fan inlet and exit points when the fans are turned off, closing off a route for CO2 gas to escape from the grow room.

 

This is pretty much the grow room design that I have been using for many years because it allows me a high degree of flexibility in that not only can it be fully sealed/enclosed but I can also occasionally vented during the lights out (night) hours when plants consume oxygen and release CO2. To touch on why I approach things this way…

 

The Night Cycle, Respiration and Oxygen

 

At night, in the absence of light, photosynthesis essentially ceases and respiration is the dominant process.

 

Respiration consists of a series of reactions that occur primarily within the mitochondria (the powerhouse of the cell) of plant cells. Plants take up oxygen through the stomata in their leaves and through their roots during respiration.

 

Respiration converts oxygen and the sugars generated during photosynthesis into carbon dioxide, water and energy. The rate of respiration for most plants peaks around the normal oxygen level in the atmosphere. Respiration decreases with decreased available oxygen.

 

In many grow rooms plant biomass in the environment can represent a high percentage of the grow rooms volume/space. For example, in a small grow room the plants in the room could be taking up to 80% of the available volume/space of the environment. What this means is that the available oxygen to plant mass is relatively low if air isn’t replaced intermittently. While CO2 contains O2 (what plants take in through their leaves/stomata and roots for respiration) O2 doesn’t disassociate from the CO2 molecule because of the strong bonding between carbon and oxygen, unless extremely high temperatures are present (such temperatures are only available in solar furnaces etc). What this really means is that CO2 can’t be used by a plant as a source of oxygen.

 

Given this, even though plants produce about ten-times as much oxygen during the day as they consume at night, the night-time consumption of oxygen by plants in confined spaces (high plant mass per volume of room space) can potentially create low oxygen conditions for respiration at night. As plants consume oxygen and release CO2 during the night, in a well-sealed grow room this potentially creates a deficit of oxygen – oxygen that is needed by the plant for respiration.

 

Based on this, I vent the grow room occasionally at night to flush out CO2 and bring in oxygen from outdoors. Additionally, having a sealed grow room which is also equipped for venting allows me to flush the grow room of CO2 and bring in oxygen immediately after the lights switch off (beginning of night cycle). This ensures adequate levels of oxygen are available to the plants from the beginning to the end of the night cycle.

 

Some growers choose to have fully sealed environments. That is, some growers choose to have environments where venting doesn’t take place at all (day or night).

 

I’d note that one significant advantage of a fully sealed growing environment is that because air isn’t being drawn in from outside, at all, this greatly reduces the chances of fungal pathogens (e.g. Botrytis and mycotoxin producing fungi such as Aspergillus) entering the growing environment; i.e. because outside air isn’t being drawn into the grow room, at all, airborne fungal pathogens that are naturally present in nature/outdoors can’t find their way into the grow room through inlet air bought in from outdoors. This minimises the chances of fungal diseases in crops. Additionally, due to not venting the grow room pests found outdoors cannot find their way into the growing environment through inlet air that is drawn in from outdoors. This acts to stop pests entering the grow room through inlet ventilation. By keeping pests outdoors through not drawing inlet air into the grow room from outdoors, this greatly reduces the chances of encountering pest problems during the crop cycle. However, on the downside not venting intermittently at nights may create an oxygen deficit which affects respiration.

 

Alternatively, venting night or day can provide pathogens and pests with a way in to the growing environment. To counter this and provide oxygen for respiration while also eliminating the risk of introducing fungal pathogens and pests into the grow room growers can filter the inlet air through HEPA (High-Efficiency Particulate Arresting) filters. HEPA filters provide a very high level of protection against airborne pathogens and 100% protection against pests. Commonly sold greenhouse HEPA filters claim to filter 90% of particulate matter which includes bacteria and fungi. See following illustrations of HEPA filter.

 

hepaunit2_joomla

 

 

HEPA filters can be used on inlet air vents/fans to effectively filter pests and airborne pathogens from inlet air. The approximate cost of a greenhouse HEPA filter (on left) is $80 – 300 USD (dependent on size). This particular brand (pictured) can be washed to ensure good performance for many years of use. Speak to your hydroponic supplier for more information about HEPA filters.

 

Be warned that any impedance (e.g. filters) on a fan reduces airflow. This needs to be compensated for. Where using HEPA filtration the ideal fan type is a centrifugal fan. Centrifugal fans build air pressure (unlike cheaper inline fans) and therefore deal with filter and ducting impedance quite effectively.

 

Optimum ppm of CO2

 

Because photosynthesis increases with high light levels, the optimal CO2 concentration becomes higher where high light levels are present. This means that in under HID light growing situations where HID lighting emits high levels of PAR (photsynthetically active radiation) optimal CO2 rates are typically higher than those expressed for greenhouse agriculture where natural light or natural light and some supplemental lighting may be used.

 

Other than light levels, the ideal level of CO2 varies with crop, temperature, RH, stage of crop development and nutrient level.

 

It is important to note that CO2 enrichment will be of most benefit in environments where nutrients, RH and ambient air temperatures are within their ideal ranges. The further away these factors are from these ideal ranges the less growth benefits elevated levels of CO2 will provide.

 

For the majority of C3 plants net photosynthesis increases as CO2 levels increase from 340 to1,000 ppm. Most crops show that for any given level of PAR, increasing the CO2 level to 1,000 ppm will increase the photosynthesis by about 50% over ambient CO2 levels.

 

Optimum ppm of CO2 in Indoor, Under Lights Environments

 

Studies with C3 crops grown in high light conditions, in general, show that 1000 to 1250ppm of CO2 is about optimum and additional CO2 doesn’t offer any growth benefits.

 

Inline to this, I have found that optimum CO2 ppm during flower is about 1100 – 1200ppm under high light conditions where environmental conditions (temp, RH, nutrition) are within ideal parameters.

 

Optimum ppm of CO2 is probably Genetic Specific

 

Research has shown that even in a single species of plant, genetic variations determine the growth outcomes elevated levels of CO2 can elicit. For example, research with Cannabis Sativa L. (2011) that trialed several sets of genetics ((high Potency Mexican variety, and K2, MX and W1 from Switzerland) under elevated CO2 and HID lighting showed the effects of elevated CO2 concentrations on Cannabis sativa varieties were significant. However, the magnitude of the increase was ‘variety-specific’.[9] See following data.

 

 

Parameters CO2 Levels (μmol mol−1) Cannabis varieties
HPM K2 MX W1
Ci/Ca 390 0.58 ± 0.06a 0.58 ± 0.07 a 0.57 ± 0.04 a 0.60 ± 0.08 a
545 0.66 ± 0.05 a 0.59 ± 0.05 a 0.61 ± 0.06 a 0.57 ± 0.05 a
700 0.64 ± 0.08 a 0.56 ± 0.08 a 0.57 ± 0.05 a 0.61 ± 0.07 a
Ci/gs 390 1.12 ± 0.14 a 1.02 ± 0.15 a 1.22 ± 0.18 a 1.03 ± 0.16 a
545 2.19 ± 0.19 ab 1.62 ± 0.13 ab 2.31 ± 0.27 ab 1.15 ± 0.13 ab
700 3.84 ± 0.22 b 3.36 ± 0.26 b 3.15 ± 0.25 b 2.48 ± 0.21 ab

 

Source: Chandra, S. Lata, H. Khan, I. A and ElSohly, M. A. (2011) Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2.

 

It is possible that some genetics could benefit from even higher levels of CO2 than I have found to be optimum and some may even realize maximum growth potential at lower ppm. However, as a rule, the machinery of photosynthesis, the chloroplast, has its limits and more CO2 simply goes to waste. I.e. a photosynthesizing organism is a tiny factory.  CO2 and H2O go in, energy is consumed, sugars are stored and H2O and O2 come out.  The processing limits of every such organism as a factory are set by the number of processing units, the amount of CO2 and H2O and the amount of light and nutrients available. If a plant has adapted to process CO2 within a particular range of values then it is unlikely it will benefit from CO2 above that range. As a result there comes a point where a critical mass is reached and more CO2 simply goes to waste.

 

Too Much CO2 is Detrimental to Growth

 

It is important to note that excess CO2 is detrimental to net photosynthesis and growth.[10] In short, too low CO2 level limits growth but too high level of CO2 is not beneficial either. Plants are more sensitive towards high CO2 concentrations than humans and show damage like burnt leaves where CO2 levels are excessive.

 

For this reason while some careful experimentation is always advised re your plant genetics don’t go too far over the 1200ppm range and certainly never exceed 1500ppm. As a rule with any experimentation always tread gently; i.e. don’t make dramatic adjustments/changes to theoretical optimums. I stress….Carefully! Carefully!

 

 

CO2 Enrichment and Plant Nutrition

 

Where plant nutrition becomes a limiting factor the benefits of CO2 enrichment will be negatively impacted.

 

It is important to note, depending on the crop type/species and the growing methodology, the increased growth rates related to CO2 enrichment will require that the nutrient solution to be applied at a somewhat higher electrical conductivity (EC) than would be considered ideal in non- CO2 enriched environments.

 

For example, elevated CO2 is shown to increase N requirement.[11] Sucrose and starch concentration generally increase in the leaves of plants grown at above- ambient CO2 and this in part explains a decrease of leaf N concentration in elevated CO2 environments – a dilution effect.[12] Other than this, stomatal conductance is altered in elevated CO2 and transpiration is decreased which some authors believe reduces N mass flow.

 

Additionally, a decrease in transpiration (due to decreased stomatal conductance) under elevated CO2 reduces calcium (Ca) and boron (B) uptake which may affect flower/fruit quality. Increased applications of these nutrients, within reason, will adequately compensate for decreased uptake.

 

Further, P demand increases due to elevated levels of CO2; i.e. several studies have reported that both the magnitude and the direction of the growth response of plants to elevated CO2 depend on P availability.[13], [14] Elevated CO2 is likely to affect the internal P requirement of plants because elevated CO2 alters P utilization within plant tissues (leaf phosphorus demand is thought to increase with increasing CO2).[15]

 

To dumb this information down somewhat, as a general rule when running a CO2 enriched environment run your EC at 0.2 – 0.4 higher than normal. E.g. if normally running an EC of 1.8 in a non- CO2 enriched environment increase this to EC 2.0 to 2.2. This is particularly relevant when using CO2 in flower when nutrient demand is high.

 

In grow, when nutrient demand is lower, run your EC at normal or just somewhat higher (0.1 – 0.3 dependent on how large you grow the plants in the 18/6 ‘veg’ light cycle).

 

Optimum Air Temperature in CO2 Enriched Environments

 

This is an important point to cover because over the years a myth seems to have persisted in the indoor grow scene that plants grown in CO2 enriched environments prefer higher ambient air temperatures than would be considered ideal in environments where CO2 enrichment isn’t used.

 

Firstly, there is some basis to this myth in that various studies have demonstrated that some crops perform better in elevated CO2 conditions at higher ambient air temperature than would be considered ideal in non- elevated CO2 environments.[16]

 

However, studies have also shown that where other crops are grown with elevated CO2 in higher than what would be considered ideal air temperatures in non-elevated CO2 conditions these crops produce lower yields and become more susceptible to disease.[17]

 

The ideal temperature in an indoor, under lights CO2 enriched environment is shown to be between 25-30oC (77 – 86oF), which is at about the same levels as would be ideal in non- CO2 enriched environments. Where temperatures exceed 30oC (86oF) growth rates begin to decline. [18] See following graphs that demonstrate photosynthetic/growth rates relative to air temperatures.

 

co2-Temp-graph

 

Source: Chandra, S. Lata, H , Khan, I. A. and Elsohly, M. A. (2008) Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions

 

Also it has been shown that at low ambient air temperatures there is little or no benefit from increased levels of CO2, but as temperatures rise towards optimum the proportionate stimulation of photosynthesis increases.

 

Therefore, the standard rule of crop and genetic specific ideal temperature ranges apply in both CO2 and non- CO2 enriched environments; i.e. although plants can exhibit some latitude with respect to temperature response of photosynthesis there is a scientific consensus that the optimum temperature for photosynthesis for an individual plant species reflects the environmental temperature range for which that species evolved.[19] In general, you will find that with most indoor genetics this temperature range typically falls between 25-30oC (77 – 86oF) during the lights on (day) period.

 

Elevated CO2 and Plant Architecture  

 

Elevated CO2 alters plant architecture through its effects on both primary and secondary meristems of shoots and roots. One outcome of this is that elevated CO2 may stimulate elongation of branches and stems without accompanying increases in the number of nodes produced.[20] This presents as an issue in indoor crops where plant stretch is often a problem that growers seek to overcome.

 

Another issue surrounding the potential of CO2 to promote stem elongation and large internode spacing (stretch) is that because CO2 enrichment promotes more vigorous growth, plants grow taller and put out lateral growth more quickly and, therefore, plant crowding can become an issue if this isn’t compensated for with adequate plant spacing. This can result in shade avoidance syndrome (SAS) which promotes stretch (see page …. for more information about SAS).

 

It is important to note that genetics play some role in the degree that CO2 enrichment can increase stem elongation and intermodal distance. However, for the most part I have found that as a general rule marginally increased internodal distance is to be expected, even when factoring in the faster growth rates of lateral branches induced by CO2 enrichment.

 

As such, after using CO2 for many years, I use it during the growth phase (at 600 – 800ppm), cease use when I switch over to 12/12 (flower) until stretch has largely completed and the buds have set and then gas up the environment to 1100 – 1200ppm until about a week a half (10 or so days) before the flush cycle when the bulking (filling/swelling) phase has finished and growth naturally slows (flowers/fruit are putting on density but little more in the way of size).

 

In other cases, many growers only increase CO2 levels during the swelling/bulking phase (i.e. they don’t gas during the veg and stretch phase).

 

CO2 and Relative Humidity

 

High relative humidity (RH) reduces transpiration in plants.

 

Transpiration is important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf. This may seem like a big temperature difference but to put it into perspective around 90% of a healthy plant’s water uptake is transpired while only around 10% is used for growth. Additionally, transpiration plays an important role in maintaining/regulating osmotic pressure in plants and enables mass flow of mineral nutrients and water from roots to shoots. Therefore, where transpiration is reduced, extremely important growth processes are affected resulting in a reduction in growth. High humidity also creates more ideal conditions for disease (e.g. fungal pathogens) to take hold in crops.

 

Excessive humidity becomes important when discussing plants being grown in elevated CO2 because high humidity can become an issue where inadequate dehumidification, through the use of a dehumidifying unit, of the grow environment exists. For this reason it is important to have a dehumidifier in your grow room that is capable of removing enough water vapour from the air to maintain the environment at 55% or lower RH at all times. I’ll talk more about this in a moment when discussing optimum RH (55%) during flower.

 

Studies have shown that excessive humidity reduces growth rates in CO2 enriched environments. For example, in cotton plant, the enhanced dry matter yield due to doubled CO2 concentration was 1.6-fold greater at low humidity than at high humidity.[21] Based on this, it is important for growers to understand that higher than optimal humidity can severely impact on growth in CO2 enriched environments.

 

The ideal humidity range for non- CO2 enriched, indoor, under light crops while varying at the different growth stages can be expressed as being somewhere between 45- 65% during flower. For myself, I have found that when growing in CO2 enriched environments where optimum temperatures and nutrition are maintained, 45 – 55% RH is about ideal during bloom – 55 ± 3% RH being about optimum during the swelling/bulking phase and 45 – 50% during late flower.

 

Optimum RH in a CO2 enriched environment (guidelines):

 

Settling Phase = 75%

 

Grow (rapid veg) = 65%

 

Mid Flower = 55 ± 3%

 

Late Flower = 45 – 50%

 

During stretch when CO2 enrichment isn’t applied = 60%

 

The Importance of Air Movement in CO2 Enriched Environments

 

A paper that is over 60 years old shows that in windless conditions CO2 concentrations over a cornfield build up each night as CO2 diffuses from higher air and the organic matter and bacteria create CO2 from the soil.  The paper by Chapman et al, from 1954[22], shows that as soon as the sun comes up, to power-up dormant photosynthetic cells, plants rapidly consume as much CO2 as possible and when the CO2 levels fall too low plant growth slows.

 

On a windless day CO2 values rose to 410ppm overnight and fell to 210ppm during the morning.

 

This data shows CO2 content of the air over a cornfield on a still day (no wind). Sunrise occurs at 5am and CO2 levels plummet reaching their lowest levels by 1pm which is nearly half the CO2 concentration of the peak reached overnight. The corn is affecting CO2 levels in air even as high as 150m or 500ft above.

 

What this study demonstrates is that the CO2 directly around plants is rapidly consumed and where air movement doesn’t supply additional CO2 the ppm of CO2 directly available to plants for photosynthesis quickly drops.

 

An important point to note also is that CO2 is heavier than other constituents of air. Therefore, the best method of delivering CO2 to the plants is by releasing the gas directly over the top of them. After this, adequate air movement within the environment is essential. You’ll note in the CO2 grow room design on page …. that a CO2 delivery hose line is placed over the tops of the lamps, well above the plants. This ensures the gas is delivered above the plants whereupon it falls directly onto/over the plants. Other than this, you will see a makeshift CO2 redelivery system on the floor that ensures CO2 is circulated at ground level back up to/through the plants.

 

This redelivery system is simply a piece of ducting with a fan attached one end and the other end blocked to stop the exit of air. I then punch some holes in the top of ducting so the air/ CO2 is pushed upwards from the floor to the plants leaf undersides (directly to the stomata). See following image of makeshift CO2 delivery system.

 

CO2-redelivery-system

 

CO2 Delivery System: Connect 3mtrs of 100mm ducting to a small fan (approx. 30Ls/sec) and block off the end of the ducting to stop air from exiting. Punch small holes randomly in the ducting to allow air/CO2 to escape from these holes. Position/spread the ducting under the plants, at floor level. This creates a gentle upward breeze, drawing air/CO2 from ground height and delivering it directly to the undersides of the plants to the stomata.

 

Keep in mind that I mentioned the CO2 redelivery system earlier when discussing higher canopy humidity levels and this encouraging plant stretch. Therefore, another benefit to the CO2 redelivery system is that it helps to keep canopy humidity levels down.

 

Additionally, there is an pedestal fan in the room to keep air moving around. Air movement ensures that the area directly around the plants doesn’t become CO2 depleted while also ensuring the CO2 sniffer/doser unit is monitoring levels of CO2 that are accurate to the room value as a whole. I.e. air movement ensures CO2 is evenly distributed throughout the plants and the room.

 

Bottom line – air movement is extremely important in any optimized CO2 grow room. Mix it up!

 

 

Equipment Required in the CO2 Enriched Grow Room

 

The right way to handle CO2 enrichment in the grow room involves using bottled, compressed CO2, a CO2 gas solenoid release valve, a CO2 sniffer/doser unit (that doses based on a set CO2 ppm value and monitors the CO2 levels in the grow environment, redosing as required), a dehumidifying unit, a refrigerated split system air conditioner (AC) and fan baffles.

 

It is important to note that growers take various approaches to producing CO2 gas, such as employing systems that mix acid and carbonates, or using dry ice or propane burners to create CO2. However, in all cases these systems have issues and for this reason I do not recommend any other means of providing CO2 gas in a well-controlled CO2 enriched indoor grow room environment other than through the use of compressed CO2 gas. For example, propane gas burners which are commonly used to produce CO2 in agricultural greenhouse settings, other than producing CO2 also produce both heat and humidity. Approximately one pound of water is added to the atmosphere for every pound of propane gas that is used. Additionally, these units are essentially high powered gas heaters that produce a lot of heat which is the last thing you need in an environment where air ventilation isn’t employed. Therefore, these units may be suitable in extremely cold climates (where heating beyond HID lamps is beneficial) and where dehumidification is adequately handled; however, in warmer climates their use could prove highly problematic where maintaining optimum air temps is concerned. I’ll cover more about the problems associated to CO2 gas producing alternatives later.

 

Anyway, let’s look at the equipment that is required to get CO2 enrichment right.

 

Equipment Needed:
CO2 Gas Bottle

 

CO2-gas-bottle

 

The most efficient and accurate means of supplying CO2 to the environment is through the use of compressed CO2 gas; i.e. CO2 gas bottles.

 

Different grades of CO2 gas are available. Always ensure that you are using pure CO2. Food grade CO2 is always pure CO2. However, refrigeration and beer grade CO2 generally contain nitrogen along with CO2 gas. For this reason the use of refrigeration and beer grade CO2 should be avoided. Check with your supplier about product options.

 

Flow Control Regulator Unit with Solenoid

 

CO2-flow-control-valve

 

A flow control regulator unit is required for releasing gas from the compressed CO2 gas bottle at the required dosage rate.

 

These units consist of a solenoid and valve and when switched on via a CO2 sniffer/doser (see following) release CO2 gas from the gas bottle until the sniffer/doser identifies that the desired ppm of CO2 is in the atmosphere and then switches the solenoid off.
CO2 Sniffer/Doser Unit

 

CO2-sniffer

 

An environmental CO2 sniffer/doser unit that measures atmospheric CO2 levels will be required to ensure that the CO2 levels are optimized and maintained in the environment at all times. These units monitor/measure the ppm of CO2 in the environment and when levels fall beyond a programmed optimum, the unit re-gasses the environment via turning on the CO2 flow control regulator which is attached to the gas bottle.

 

As a warning, there are units on the market that simply operate as timers for re-gassing without the sniffer/measuring function. These units, while cheaper than automated sniffer/doser units, do not automatically maintain optimal atmospheric ppm of CO2 in the environment.

 

Without a sniffer you will need to manually sniff the air for ppm of CO2 with a handheld sensor that doesn’t re-gas automatically (it simply tells you how much CO2 is present at the time of measurement). Anyone who has done this with plants that consume more and more CO2 as they grow will attest to the fact that this method of monitoring CO2 is highly unreliable as a means to maintain optimum ppm of atmospheric CO2 at all times.

 

For what it is worth, in order to ensure absolute control over the environmental ppm of CO2, invest wisely and purchase a unit with a sniffer.

 

Air-Cooled Shades or Tubes

 

air-cooled-shade

 

HID lamps create a lot of heat. This can become incredibly problematic in non-vented environments where cooler outside air isn’t being drawn into the grow room to compensate for lamp heat.

 

Therefore, in order to control the grow room temperature it is necessary to remove lamp heat from the environment without removing the CO2 enriched air.

 

Air-cooled shades and tubes fully enclose the lamp in a sealed shade or a glass tube that surrounds the lamp. Air is then drawn through the shade or tube to draw off lamp heat. Air passes through the shade or tube, removing a percentage (50% – 95%) of the lamp heat with it (cooling levels will rely heavily on the volume of air passing through the shade or tube).

 

Through the use of air cooled shades you can then remove lamp heat without removing any of the air/CO2 from the grow room environment.

 

Fan/s for Air-Cooled Shades    

 

Can-Fan-site

 

Ducting is attached to air cooled shades or tubes and a fan is used to move air through this ducting. The ducting is channeled outdoors where the hot air from the lamps is released outside of the grow environment.

 

To reassert a point, in warmer environments the more lamp heat removed the better. Therefore, where growing in warm climatic zones high volume centrifugal fans are recommended for efficiently moving large amounts of air through the air cooled lamp devices. The more air you move, the more heat that is removed from lamps before this heat is able to enter the grow environment. Conversely, in cold climatic zones lamp heat can be used to warm the grow environment so high air movement isn’t of so much importance in cold climate situations. In the past, where I have grown in cold climates I have configured my air cooled lamp system (fans) with a thermo fan controller to turn on and off based on the grow room temperature. Using this configuration I am then able to use my HID lights not only for light but for heating.

 

You can configure one fan to air cool multiple lights using adapters. Speak to hydroponic supplier about fan requirements and adapters.
Split System Air Conditioner

 

Split-AC
Split system refrigerated air conditioners (ACs) are the ideal for the CO2 enriched grow room. ACs that recycle the environment’s air (while cooling) are a must; i.e. ACs that draw air from outside (rather than recirculate the grow room air) will reduce CO2 levels in the grow room quickly. As such, ACs other than split system ACs should not be used in CO2 enriched environments.

 

An AC enables us to control heat buildup that can occur due to outside air temperatures and heat from lamps (that is emitted, to some degree, even with air cooled lamps in use). Ideally, the air conditioner is a refrigerated unit, as water based units increase the relative humidity levels within the growing environment (which is not desirable). The ideal power requirements of the air conditioner (how many watts/HP) will depend on your room size, outside ambient air temps, the numbers of lamps and the efficiency of which these lamps are cooled by air cooling.

 

In warmer climates, because ACs draw more power than fans, it is highly recommended that you use quality centrifugal fans with high airflow ratings to remove as much lamp heat as possible before it enters the grow environment., i.e. the more lamp heat removed prior to it entering the grow environment the less AC (and power draw) needed for compensating for lamp created heat. Speak to your hydroponic supplier for further information about this.

 

Dehumidifier

 

dehumidifier

 

In order to reduce humidity in the grow environment a dehumidifying unit is an absolute must.

 

Dehumidifying units typically operate by drawing air across a cold surface. Since the saturation vapor pressure of water decreases with decreasing temperature the water in the air condenses on the surface, separating the water from the air.

 

The problem is that in order to cool something you also need to create heat (action/reaction). Think of this as what occurs with a refrigerated AC. That is, there is a reason that the cooling fins and compressor of an AC are placed outdoors – they produce heat. Dehumidifiers operate in a very similar manner to ACs. Therefore, dehumidifiers also add some heat to the grow environment. The smaller the room, the more the air temperature will increase while in a large area the effect is usually very little. However, this heat needs to be offset with AC.

 

Most portable dehumidifiers are equipped with a water collection receptacle, typically with a float sensor that detects when the collection vessel is full to shut off the dehumidifier and prevent an overflow of collected water. In grow environments, where plants are transpiring lots of water these receptacles will generally fill with water in 4-6 hours and may need to be manually emptied and replaced several times per day to ensure continued operation. Emptying the collection vessel can be a hassle and if you aren’t around, or fail to exercise due caution (i.e., check regularly), the unit can shut off. Humidity in the grow room then rises quickly. However, many portable dehumidifiers can also be adapted to connect the condensate drip output directly to a drain via a hose. If this isn’t the case, what I typically do is modify the collection vessel by drilling it and run a hose from it that can then channel the water to a drain or outdoors. This way the collection vessel never fills and the dehumidifier never automatically shuts down.

 

The effectiveness of dehumidifiers can vary greatly re the volume of water they remove from the air. Where small household dehumidifiers are concerned, these units were never developed for grow room conditions where large numbers of transpiring plants along with other sources of water vapour (e.g. irrigation) are present. For this reason, dependent on the size of the grow room and the numbers of plants etc you may find that if using a household dehumidifier, 2 or more units are required to maintain optimum RH. There are larger industrial units also available that are more robust and efficient for larger scale applications. Speak to your hydroponic supplier for further information.

 

Running Your CO2 Enriched Environment

 

Environment is king!

 

We’ve pretty much covered it all where optimums are concerned re running a CO2 enriched grow room. To put things simply, the critical thing in running your CO2 grow room is to ensure that all environmental parameters are maintained at optimum for as much as the time as absolutely possible. This means attention should be paid to RH, ambient air temperatures, nutrient levels and atmospheric levels of CO2.

 

While some growth benefits will likely be realized in a less than ideal environment, numerous studies show that where environmental optimums are maintained net photosynthesis and growth rates are dramatically increased.

 

Where the CO2 gas itself is concerned, the trick is to contain the CO2 in the environment through not extracting air during the day and through ensuring the environment is tightly sealed to minimize CO2 leakage.

 

As the plants consume CO2 and release oxygen, the CO2 is maintained at optimum by a CO2 sniffer/doser unit which re-gasses the environment inline to a set atmospheric ppm value that can be manually dialed in on the unit. So, for example, during grow you enter e.g. 800ppm and the unit maintains atmospheric CO2 at 800ppm; in bloom you enter 1200ppm and the sniffer/doser unit maintains atmospheric CO2 at 1200ppm. That part, actually, is dead simple if you are using the right equipment. When using the wrong equipment (e.g. a CO2 timer unit with no sniffer) things become far more complex and far less reliable; i.e. without a sniffer you will need to manually measure the air for ppm of CO2 with a handheld sensor that doesn’t re-gas automatically (it simply tells you how much CO2 is present at the time of measurement). Anyone who has done this with plants that consume more and more CO2 as they grow will attest to the fact that this method of monitoring CO2 is highly unreliable as a means to maintain optimum ppm of atmospheric CO2 at all times.

 

One thing to be mindful of when setting up your CO2 grow room is to factor in worst case scenarios. So, for example, if during winter (cool/cold outside air temperatures) you find that you’re only just able to maintain air temperatures at a minimum of 28 – 300C, then come summer when outdoor temperatures rise you’ll probably find the grow room temperature becomes excessive using the same grow room configuration/hardware. Based on this, always try to configure your grow room for the worst and not the best case scenario. This will save you money in the long-term. Put simply, upgrades can be costly.

 

As has been noted, some growers run a fully sealed environment where air exchange never takes place, night or day. However, plants require oxygen at nighttime for respiration. For this reason it is advisable that growers intermittently vent their grow room 15 minutes of every hour or two over the course of the night. Additionally, the room should be fully vented for at least 15 – 30 minutes after the lamps turn off (beginning of night cycle) to remove CO2 and bring in oxygen.

 

A factor to consider with night venting is the temperatures outdoors (remember you are drawing air from outdoors). For instance, if your desired optimum grow room night temperature was 200C (680F) and outdoor temperatures were extremely low (e.g. 00C ) you would be better off venting less frequently than if outdoor temperatures were say 180C (64.40F). In a cold weather scenario 15 minutes every 3 – 4 hours should suffice. Additionally, it would be ideal to have some heating come on during and after venting to bring the room temperature back to optimum as quickly as possible. In the past I have used electric heating and thermo control units for this purpose. Speak to your hydroponic supplier about options here.

 

Conversely, in extremely hot environments similar considerations should be factored in to your night-time venting regime. For example, in places like Australia where in some areas night temperatures can hover around the high 30s to even low 40s (0C) venting should be done for 15 minutes every few hours. Your AC will then bring the room back to its desired temperature reasonably quickly and in between venting your room temperature should remain at optimum for the majority of the night.

 

Where temperatures outdoors are close to your grow room night temperature optimum vent 15 minutes every hour or two.

 

By the way, venting requirements will depend somewhat on plant to room/air volume so this should be considered in your night venting routine. For example, if you have an inlet air fan configuration that only draws in low amounts of air you might want to vent for 30 minutes rather than 15 minutes. Conversely, if your inlet fan/s draw in higher amounts of air, 15 minute inlet air fan cycles will suffice.

 

Remember also that air movement within the grow environment is important to ensure that the CO2 is circulated around/throughout the environment. CO2 redelivery systems on the floors and oscillating fans, therefore, should be used.

 

Using CO2 during Veg

 

Many growers employ short vegetative cycles (e.g. 5-10 days), and in a lot of cases this cycle is only used to settle the plants in and get them to the vigorous veg stage before switching the plants down into the 12/12 (bloom) light cycle. In other cases such as SOG growing there may be no veg stage at all (i.e. clones are placed directly into the 12/12 light cycle) or the plants may be ‘vegged’ for just a couple of days. Where short vegetative cycles are employed and because of the cost of CO2 gas there is perhaps little point in elevating CO2 during this period. Where larger plants are being grown and the vegetative stage is longer, elevating CO2 definitely has its advantages.

 

Using CO2 during Stretch

 

Going back to the material we have covered on plant elongation/stretch it can be beneficial re plant architecture to slow down photosynthetic rates during stretch in order to minimise plant elongation. As elevated levels of CO2 increase photosynthetic rates it is advisable not to CO2 enrich the grow environment during the stretch phase.

 

Gas Up the Grow Room 30 – 40 minutes Before the Lights Come On

 

This one pretty much speaks for itself. Begin gassing up the grow room before the lights come on. The most vigorous growth of plants tends to occur within the first few hours of lights on. Having lower than ideal levels of CO2 at any time during this period, therefore, impacts on photosynthesis during the most vigorous stage of growth.

 

Gas only During the Lights on Period

 

Based on the theory that plants take in CO2 and release oxygen during the day while they take in oxygen and release CO2 at nights you can perhaps understand that CO2 enrichment should only be applied during the lights on period when plants are photosynthesizing.

 

Air pumps used for Aeration of the Nutrient Solution in CO2 Enriched Environments

 

There have been claims made by some that air pumps in CO2 enriched environments should be kept out of the grow room because elevated levels of atmospheric CO2 (taken in by air pumps which deliver the grow room air/CO2 to the nutrient solution) results in significant pH shifts in the nutrient that is being provided to the plants. There is some basis to this claim, albeit it is not too much of an issue.

 

The science…

 

Aqueous dissolved carbon dioxide, CO2 (aq), reacts with water forming carbonic acid, H2CO3 (aq). Carbonic acid may lose protons (removal of H+) to form bicarbonate, HCO3, and carbonate, CO32-. In this case the proton is liberated to the water, decreasing pH. However, CO2 gas dissolves slowly in water where about 1% of it forms carbonic acid.[1] Carbonic acid then is partially ionized, forming HCO3– and CO32-. About 99% of CO2 dissolved in water remains as dissolved CO2 gas. Much of this gas finds its way back into the grow room atmosphere through the substrate solution.

 

One other claim I have encountered is that air pumps should be removed from a CO2 enriched environment because air containing elevated CO2 can cause root disease. However, I have never found this to be the case. Additionally, there is no scientific basis to support this claim. In fact, scientifically speaking, possibly the opposite is true.

 

Studies have been conducted on whether CO2 delivered to the roots of plants increases growth rates. The findings from these studies are variable and while a definitive answer is illusive, various aspects of the issue have been described by Idso (1989) with:

 

“Although several investigators have claimed that plants should receive little direct benefit from dissolved CO2 (Stolwijk et al., 1957; Skok et al., 1962; Splittstoesser, 1966), a number of experiments have produced significant increases in root growth (Erickson, 1946; Leonard and Pinckard, 1946; Geisler, 1963; Yorgalevitch and Janes, 1988), as well as yield itself (Kursanov et al., 1951; Grinfeld, 1954; Nakayama and Bucks, 1980; Baron and Gorski, 1986), with CO2-enriched irrigation water.  Early on, Misra (1951) suggested that this beneficent effect may be related to CO2-induced changes in soil nutrient availability; and this hypothesis may well be correct.  Arteca et al. (1979), for example, have observed K, Ca and Mg to be better absorbed by potato roots when the concentration of CO2 in the soil solution is increased; while Mauney and Hendrix (1988) found Zn and Mn to be better absorbed by cotton under such conditions, and Yurgalevitch and Janes (1988) found an enhancement of the absorption of Rb by tomato roots.  In all cases, large increases in either total plant growth or ultimate yield accompanied the enhanced uptake of nutrients.  Consequently, as it has been suggested that CO2 concentration plays a major role in determining the porosity, plasticity and charge of cell membranes (Jackson and Coleman, 1959; Mitz, 1979), which could thereby alter ion uptake and organic acid production (Yorgalevitch and Janes, 1988), it is possible that some such suite of mechanisms may well be responsible for the plant productivity increases often observed to result from enhanced concentrations of CO2 in the soil solution.”[2]

 

Basically, leaving the air pump in the grow room shouldn’t pose any problems. Certainly, after 15 or more years of growing under lights in a CO2 enriched environment, I leave the air pump in the grow room.

 

This said, no harm can come from placing the air pump outside of the grow room. Therefore, either option is fine.

 

Alternatives for Producing CO2 Gas

 

Products are available that release CO2 into the atmosphere via chemical combinations/reactions (hydrochloric acid and bicarbonate soda etc). In the past where I have looked over these products I have concluded; 1) often they are dangerous – mixing bicarbonates and hydrochloric acid can be a messy and dangerous affair and; 2) there is no actual way of releasing ideal levels of CO2 into the environment. For what it is worth, give these products a wide berth.

 

Other common methods used by growers to enrich the atmosphere with CO2 are:

 

  • Brewing beer in their grow room (a by-product of fermentation is CO2)
  • Gas heating (CO2 is created by combusting propane gas)
  • Placing dry Ice in the environment (As it breaks down/melts dry ice releases CO2)
  • CO2 Typically something along the lines of a mix of citric acid and carbonates that when added to water release small volumes of CO2 gas.
  • Multiple grow rooms where some are at night cycle and some at day cycle and where CO2 is drawn from the night cycle rooms into the day cycle rooms.

 

OK, so that sounds cheaper and easier. Do any of them work?

 

In a nutshell – no! Not unless it is possible to contain the CO2 enriched atmosphere within the growing environment for extended periods of time and maintain it at ideal ppm levels.

 

Brewing Beer releases only small amounts of CO2. This means that it would take forever for the CO2 levels to reach 1200ppm (providing the plants weren’t actually using it).

Gas heating is fine for greenhouses that are located in cooler climates. In the grow room they create far too much heat. Propane burners also add humidity to the atmosphere. Approximately one pound of water is added to the atmosphere for every pound of propane gas that is used.

Dry Ice is an expensive product given the amounts that you would need to provide enough CO2. In addition to this, dry ice is nearly impossible to store – it even breaks down in the freezer. This means that you would be visiting your dry ice supplier on a daily basis.

CO2 Crystals: While it is possible for the manufacturers to provide a reasonably accurate CO2 ppm to room size dosage rate, the use of this type of product would prove expensive when compared to compressed CO2 gas. This method also lacks automation, which means that it would be necessary for you to manually dose every half an hour (or less).

 

Multiple grow rooms in alternating light cycles: Plants release about 10 times more oxygen than CO2 – unless you had a massive grow room in its night cycle feeding onto a very small grow room (or closet) there is no conceivable way that you can provide anywhere enough CO2 using this method – a highly flawed system at best.

 

Furthermore, all of these methods lack the necessary gas release/dosage control that is ideally required.

 

For what it is worth if you are going to create a CO2 enriched environment invest in gas bottles, regulators and a CO2 sniffer/dosage unit. This type of equipment has become the standard method of gassing in indoor growing situations for a good reason… it allows automation and fine control.

 

CO2 and Your Health

 

As a general rule, exposure to 5000ppm (or more) of CO2 is potentially hazardous to your health. This figure is based on eight hours, or more, of exposure.

 

One Last Thing

 

Now that you’ve covered this material, you might be excited by the idea of gassing your room and growing hard. That’s great! The only thing is, if you are just setting out on your indoor growing journey take a breath and hold off for just the mo. It’s going to be hard enough getting it right without this level of sophistication. Remember KISS. Baby steps first!

 

References

[1] Zelitch I (1975). Improving the efficiency of photosynthesis. Science, 188: 626-633.

[2] Leakey ADB and Lau JA (2012). Evolutionary context for understanding and manipulating plant responses to past, present and future atmospheric [CO2]. Phil. Trans. R. Soc. B., 367: 613– 629.

[3] Karowe, D. N. and Grubb, C (2011) Elevated CO2 increases constitutive phenolics and trichomes, but decreases inducibility of phenolics in Brassica rapa (Brassicaceae).

[4] Ghasemzadeh, A. et al. (2010) Elevated Carbon Dioxide Increases Contents of Flavonoids and Phenolic Compounds, and Antioxidant Activities in Malaysian Young Ginger (Zingiber officinale Roscoe.) Varieties

[5] Cheng, W. (1998) Rhizosphere feedback in elevated CO2

[6] Drigo B, van Veen JA, Kowalchuk GA, (2009) ‘Specific rhizosphere bacterial and fungal groups respond differently to elevated atmospheric CO2‘, ISME Journal, vol.3, pp 1204-1217

[7] Agricultural Dimensions of Global Climate Change. Kaiser, H. M. Drennen, T. E. (Eds). St.Lucie Press: Florida, 1993, pp 158

[8] Agricultural Dimensions of Global Climate Change. Kaiser, H. M. Drennen, T. E. (Eds). St.Lucie Press: Florida, 1993

[9] Chandra, S. Lata, H. Khan, I. A and ElSohly, M. A. (2011) Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2. Physiol Mol Biol Plants. 2011 Jul; 17(3): 291–295. Retrieved 25/5/15 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3550578/

[10] Hicklenton, R. Jolliffe, P. A. (1980) Alterations in the physiology of CO2 exchange in tomato plants grown in CO2-enriched atmospheres

[11] STITT, M & KRAPP, A. (1999)The interaction between elevated carbon dioxide and nitrogen nutrition: the physiological and molecular background, Plant, Cell and Environment (1999) 22, 583–621

[12] Wolfe, D. W. et al. (1998) Integration of photosynthetic acclimation to CO2 at the whole-plant level

[13] BassiriRad H, Gutschick VP, Lussenhop J. 2001. Root system adjustments:regulation of plant nutrient uptake and growth responses to elevated CO2.Oecologia 126: 305_320.

[14] Jin J, Tang C, Armstrong R, Butterly, C, Sale P. 2013. Elevated CO2 temporally enhances phosphorus immobilization in the rhizosphere of wheat and chickpea. Plant and Soil 368: 315–328.

[15] Niu YF, Chai RS, Dong HF, Wang H, Tang CX, Zhang YS. 2013a. Effect of elevated CO2 on phosphorus nutrition of phosphate-deficient Arabidopsis thaliana (L.) Heynh under different nitrogen forms. Journal of Experimental Botany 64: 355–367.

[16] Center for the Study of Carbon Dioxide and Global Change.  “Interactive Effects of Temperature and Enhanced CO2 on Agricultural Crops.”  Last modified May 15, 2013.  http://www.co2science.org/subject/g/summaries/tempco2ag.php

[17] Pugliese, M et al. (2012) Effects of elevated CO2 and temperature on interactions of zucchini and powdery mildew

[18] Chandra, S. Lata, H , Khan, I. A. and Elsohly, M. A. (2008) Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions

[19] Berry J and Bijorkman O (1980). Photosynthetic response and adaptation to temperature in higher plants. Ann.Rev. Plant Physiol., 31: 491-543.

[20] Pritchard, S. and Prior, S. A. (1999) Elevated CO2 and plant structure: a review

[21] Wong, S-C. (1993) Interaction between elevated atmospheric concentration of CO2 and humidity on plant growth: comparison between cotton and radish

[22] Chapman H. W .,Gleason L. S., Loomis W. E. (1954): The carbon dioxide content of field air. Plant Physiology 29,6, pp 500-503

[1] Enoch, H. Z. and Olesen, J. M. (1993) Plant Response to Irrigation Water Enriched with Carbon Dioxide. New Phytol. 125: 249 – 258

[2] Idso, S.B.  1989.  Carbon Dioxide and Global Change: Earth in Transition.  IBR Press, Tempe, AZ.